Evaporative or vaporizing heat exchangers that transfer heat from one fluid flow to a vaporizing fluid flow to vaporize the vaporizing fluid flow are known. One example of such heat exchangers is found in the fuel processing systems for proton exchange membrane (PEM) type fuel cell systems, wherein a gaseous mixture of water vapor and a hydrocarbon are chemically reformed at high temperature to produce a hydrogen-rich gas flow stream known as reformate. Typically, to produce the gaseous mixture of water vapor and hydrocarbon, these systems will use an evaporative heat exchanger to either vaporize a liquid water and liquid hydrocarbon mixture, or to produce steam from liquid water which will then be used for humidification of a gaseous hydrocarbon fuel, such as methane. In some fuel processing systems, the heat from the reformate gas flow is used to provide at least part of the substantial amount of latent heat required for vaporization of the liquid flow of the vaporizing fluid, which is advantageous because it reduces the waste heat from the system and cools the reformate to the desired temperatures required for subsequent catalytic reactions. In this regard, in some systems the optimal temperature for the preferential oxidation reaction of the reformate gas flow is roughly the same as the boiling temperature for the liquid flow of the vaporizing fluid flow which makes it advantageous to use the reformate gas flow immediately upstream of the preferential oxidizer as the heat source for vaporization of the vaporizing fluid flow, thereby cooling the reformate gas flow to the desired temperature for the preferential oxidation reaction. However, typically the sensible heat given up by the reformate gas flow is not sufficient to completely vaporize the liquid flow. One other common source of additional heat in fuel cell systems is the anode exhaust gas produced by the combustion of the anode tail gas in a catalytic reactor. It is known to use the anode exhaust gas stream in a two stage vaporization procedure wherein the vaporizing fluid flow is first partially vaporized by the reformate gas stream entering the preferential oxidizer, and is subsequentially further vaporized by the anode exhaust gas stream.
While the above described systems may work well for their intended purposes, there is always room for improvements. For example, because the heat adsorbed by the liquid is mostly latent heat, a large portion of the length of each evaporator can be occupied by a two-phase fluid. Because different flow conditions can produce the same pressure drop (for example high mass flow with low quality change or low mass flow with superheat) and can therefor coexist in parallel passages, flow distribution in such evaporators is not self-correcting. Different flow distributions can result in heat fluxes that vary significantly from passage to passage which can result in poor performance and stability. Furthermore, when multiple stages are used for vaporization, there can be difficulty in redistributing the 2-phase mixture between the two stages of vaporization.